1. Field of the Invention
The present invention relates to a system for removing solid carbonaceous residue (hereinafter referred to as “coke”) from large cylindrical vessels called coke drums. This removal process is often referred to as “decoking. In particular the present invention relates to a safety system for providing uncompromised and continuous containment for protecting operators during the entire delayed coker operation.
2. Background and Related Art
Petroleum refining operations in which crude oil is processed to produce gasoline, diesel fuel, lubricants and so forth, frequently produce residual oils that have very little value. The value of residual oils can be substantially increased when processed in a “delayed coker unit.” Residual oil, when processed in a delayed coker is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields petroleum coke, a material composed mostly of carbon. A large vessel hereafter called a “coke drum” is provided at the furnace outlet to allow sufficient residence time for the hydrocarbons to complete destructive distillation reaction. The typical coke drum is a large, upright, cylindrical, metal vessel that may, for example, be in the order of approximately 90-100 feet in height (27.4-30.4 meters) and 20-30 feet in diameter (6.1-9.1 meters), although the actual structural size and shape of the coke drum can vary considerably from one installation to another.
Typically, a delayed coking unit has an even number of coke drums. The production of coke is a batch process. Coker feedstock is deposited as a hot liquid slurry in a coke drum. Lighter hydrocarbons which are products of destructive distillation flow out the top of the coke drum. Heavier material remains in the coke drum. When a coke drum is filled, residual oil from the furnace is diverted to another coke drum. The liquid mass remaining in the coke drum cools and is quenched as a part of the process. Solid coke formed as the drum cools must be removed from the drum so that the drum can be reused. While coke is being cooled in one or more drums and while the cooled coke is being extracted from one or more drums, other drums are employed to receive the continuous production of coke feedstock as a part of the delayed coker process.
Residual oil is heated to a temperature of typically about 900° F. (477.4.degree. C.). The oil flows directly from the furnace to a coke drum. The liquid mass enters the drum, typically flowing through an opening in the bottom of the drum and, as the liquid level rises, the thermal cracking continues and layers of coke are laid down and solidify as the coke drum is cooled. Eventually the coke drum is filled substantially full with a solid mass.
When a coke drum is filled to the desired capacity, and after feedstock is diverted to another drum, steam is typically introduced into the drum to strip hydrocarbon vapors off of the solid material. The drum remains substantially full of coke that, as it cools, hardens into solid material.
It is a standard procedure to cool coke in a drum by the admission of steam then followed by water, that is, to cool the coke after the hydrocarbon vapors have been stripped off.
After a coke drum has been filled, stripped and then quenched so that the coke is in a solid state and the temperature is reduced to a reasonable level, quench water is drained from the drum through piping to allow for safe unheading of the drum. The bottom opening is uncovered, that is unheaded, to permit removing coke. Shot coke may have plugged off the drain line preventing a complete draining of the drum. Shot coke may also be loosely packed inside the drum and may “cave in” in an avalanche-like fashion and spilling onto the switch deck area below the coke drum causing substantial operating delay and creating potential hazards to personnel. Operating personnel are required to exercise reasonable caution to avoid coke hot water and hot vapors that may be released when a cave-in occurs. Procedures required to minimize the potentially harmful effects of a cave-in usually take a substantial amount of time and are not always completely effective. Once the unheading is complete, the coke in the drum is cut out of the drum by high pressure water jets. If the drum contains shot coke further avalanches my occur.
In some installations, a coke chute is located in a channel below the switch deck floor with a coke pit below it. Once the coke drum head is removed, the chute is raised to mate with the coke drum bottom flange. This process may not be completely satisfactory in that there is exposure to an avalanche of shot coke when raising the chute and the chute may be overwhelmed or may not function in the event of a cave in.
Often a pair of coke drums cycle between coking and decoking. One coke drum is coking, while the other is decoking (quenching, followed by remotely opening the joints then decoking the drum). In the decoking phase coke is removed from the coke drums by high pressure hydrostatic drilling. A drill bit is lowered into the coke drum through a drum-top deheading system and coke, cut by the drilling action, falls through a decoke chute attached to an opening in the bottom of the coke drum created when a drum-bottom deheading system removes a closure away from said opening.
Safely preparing a coke drum for decoking involves the following steps: (1) removing the working surface opening cover creating an opening in the working surface for the coke to pass; (2) remotely aligning and engaging a closure transport to the drum-bottom closure; (3) remotely energizing the drum-bottom closure to the coke drum; (4) remotely unlocking, disconnecting and separating the coke drum from the inlet pipe; (5) remotely unlocking the drum-bottom closure from the coke drum; (6) remotely disengaging the drum-bottom closure from the coke drum in a controlled manner; (7) remotely removing the drum-bottom closure from the opening in the bottom of the coke drum; (8) remotely producing and securing a passageway between the bottom opening of the coke drum to the opening in the working surface, i.e. a decoke chute; (9) remotely unlocking and moving the drum-top closure from the opening in the top of the coke drum; (10) lowering the drill bit into the coke drum through the opening in the top of the coke drum; and (11) engaging and locking the drilling head to the drum-top deheading system.
Safely preparing a decoked coke drum for coking involves the following steps: (1) remotely replacing, aligning and locking the drum-top closure to the coke drum once the drill bit is removed from the coke drum; (2) remotely decommissioning the decoke chute and replacing the working surface opening cover; (3) remotely aligning and locking the open ends of the inlet piping together, which reconnects the coke drum to the inlet pipe; (4) remotely replacing, aligning and locking the drum-bottom closure to the opening at the bottom of the coke drum.
Currently most cokers employ workers to manually perform some or all of the foregoing steps. Any of these steps can be hazardous to workers, but by far the most dangerous steps are in the transition from the coking phase to the decoking phase. Here a closed and quenched coke drum must be opened to allow the evacuation of coke from the coke drum.
Workers are most frequently harmed while performing the following steps: (1) manually unlocking, disconnecting and separating the coke drum from the inlet pipe; (2) manually unlocking the drum-bottom closure from the coke drum; or (3) manually disengaging the drum-bottom closure from the coke drum. Further workers are often harmed when pressurized gasses are released from the top of the deheading unit.
Coke is supposed to support itself in the coke drum when an opening is created at the drum-bottom; however, this cannot be assured. The flow of loose coke and quench water or other materials from other types of vessels can be very hazardous for workers performing functions during the opening of the vessels. This hazard exists until a secure passageway is present between the opening of the vessel and where the material is ultimately destined. In the case of a coking unit, the material is due to fall in a hole in a working surface located beneath the unit and towards an ultimate destination below the working surface. An even more hazardous environment is a coker design to produce “shot coke” where the coke will not support itself in the coke drum.
For all the above reasons, decoking a coke drum has been a relatively cautious and slow process especially when shot coke is produced and may expose workmen to a disagreeable and potentially dangerous environment. It is this situation to which the present invention is directed. This invention provides improved safety when working around coke drums that substantially reduces the exposure of workmen to the hazardous conditions that may be associated with unheading, the initial steps of unloading a coke drum, and the process of decoking the drum. It also benefits operations because it reduces the time required to safely return the coke drum back to service after removing the coke from the coke drum.